Thermoplastic polyesters, such as poly(1,4-butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) are a class of materials which possess a good balance of properties comprising good elongation, high strength, high energy to break and stiffness which make them useful as structural materials. However, thermoplastic polyesters are quite sensitive to crack propagation. Consequently, a major deficiency of thermoplastic polyesters is their poor resistance to impact and their tendency to break in a brittle rather than ductile manner.
In general, improvements in the impact resistance of thermoplastic resins have been achieved by incorporating a low modulus rubber. Moreover, good dispersion of the rubber phase as well as developing adhesion between the rubber and matrix contribute to efficient impact modification of these resins.
It is well known to those skilled in the art that hydrogenated block copolymers of styrene and butadiene possess many of the properties useful for impact modification of plastics. These low modulus rubber materials display a PG,3 low glass transition temperature, a characteristic advantage for optimum toughening at lower temperatures. Furthermore, these block copolymers contain little unsaturation which facilitates their blending with high processing temperature plastics without degradation of the elastomer phase.
Block copolymers are unique impact modifiers compared to other rubbers in that they contain blocks which are microphase separated over the range of applications and processing conditions. These polymer segments may be tailored to become miscible with the resin to be modified. Good particle-matrix adhesion is obtained when different segments of the block copolymer reside in the matrix and in the rubber phase. This behavior is observed when hydrogenated block copolymers of styrene and butadiene are blended with resins such as polyolefins and polystyrene. Impact properties competitive with high impact polystyrene are obtained due to the compatibility of polystyrene with the polystyrene endblock of the block copolymer. Other polyolefins are toughened due to enhanced compatibility with the rubber segment.
Although the hydrogenated block copolymers do have many of the characteristics required for plastic impact modification, these materials are deficient as impact modifiers for many materials which are dissimilar in structure to styrene or hydrogenated butadiene. In particular, significant improvement in the impact resistance of polyesters with the addition of these hydrocarbon polymers has not been achieved. This result is due to poor interfacial interaction between the blend components and poor dispersion of the rubber particles. Poor interfacial adhesion affords areas of severe weakness in articles manufactured from such blends which when under impact result in facile mechanical failure.
The placement of functional groups onto the block copolymer may provide sites for interactions with such polar resins and, hence may extend the range of applicability of this elastomer. Such interactions, which include chemical reaction, hydrogen bonding and dipole interactions, are a route to achieving improved interfacial adhesion and particle dispersion, hence improved impact modification of polar thermoplastics.
Many attempts have been made to improve the impact properties of polyesters by adding low modulus modifiers which contain polar moieties as a result of polymerization or which have been modified to contain polar moieties by various grafting techniques. To this end, various compositions have been proposed utilizing such modifiers having carboxylic acid moieties and derivatives thereof, for example, Epstein in U.S. Pat. No. 4,172,859; Saito et al. in German Offenlegungsschrift 3,022,258 (published Jan. 8, 1981); and Shiraki et al. in U.S. Pat. Nos. 4,628,072 and 4,657,971.
Epstein discloses a broad range of low modulus polyester modifiers which have been prepared by free radical copolymerization of specific monomers with acid containing monomers. Alternatively, Epstein discloses the modification of polymers by grafting thereto specific carboxylic acid containing monomers. The grafting techniques allowed for therein are limited to thermal addition (ene reaction) and to nitrene insertion into C--H bonds or addition to C.dbd.C bonds (ethylenic unsaturation). Though Epstein does disclose a broad range of polyester modifiers, Epstein does not disclose or suggest the utilization of hydrogenated copolymers of alkenyl arenes and conjugated dienes nor, more particularly, modified selectively hydrogenated copolymers of alkenyl arenes and conjugated dienes as polyester modifiers.
Saito et al. disclose thermoplastic polyester compositions which contain a modified unsaturated aromatic vinyl compound/conjugated diene block copolymer as a polyester modifier. The unsaturated block copolymer has been modified by grafting a dicarboxylic acid group or derivative thereof (e.g. anhydride moieties) at a point of ethylenic unsaturation via thermal addition (ene reaction). However, such modifiers and compositions containing same are deficient in that the weatherability and resistance to thermal deterioration are poor; and, therefore, the polymers and compositions have been used only in the fields where such properties are not required. Furthermore, it is also noted that the ene reaction is a reversible reaction.
Shiraki et al. also describe a polyester composition containing a block copolymer similar to that of Saito et al. However, in order to improve the weatherability and resistance to heat aging, Shiraki et al. partially hydrogenate the block copolymer in their respective blends to an ethylenic unsaturation degree not exceeding 20 percent of the ethylenic unsaturation contained in the block copolymer prior to hydrogenation. Once the block copolymer is partially hydrogenated, the block copolymer is modified by grafting a molecular unit containing a carboxylic acid group and/or a group derived therefrom (e.g. anhydride moieties).
As is readily apparent in each of the foregoing prior art polyester compositions utilizing alkenyl arene/conjugated diene block copolymers as polyester modifiers, improved impact modification of the particular polyester is achieved via specific interactions, between the modified diene block and the polyester.
On the otherhand, Gergen et al., in the copending U.S. Pat. No. 4,797,447, describe a polyester composition containing a block copolymer which is a thermally stable, modified, selectively hydrogenated, high 1,2 content alkenyl arene/conjugated diene block copolymer grafted with at least one functional group utilizing the metalation process. Therein, the functional groups are grafted primarily in the alkenyl arene block. In this composition, interactions between the polyester and rubber are achieved via the alkenyl arene block.
Further research and experimentation on polyester compositions containing the modified block copolymers of Gergen et al. in copending U.S. Pat. No. 4,797,447 have yielded unexpected and significant impact property improvements. These new polyester blend compositions contain block copolymers having the carboxyl functional groups present in either or any of their acid, ester and neutralized metal carboxylate salt forms. Whether either or any of these forms in combination produce improvements may be dependent on the particular polyester(s) selected. Furthermore, the impact properties are also improved by increasing the degree of carboxyl functionality.
To those skilled in the art, the degree to which the grafting reaction and particle size reduction occur, thereby promoting interfacial adhesion, together with the dispersion of the rubber within the blend typically contribute to impact toughening of the blend. The results herein demonstrate that functionalizing the alkenyl arene segment promotes covalent bonding or a strong interaction between the modified block copolymer and the polyester. Furthermore, the block copolymer also becomes well dispersed in the polyester phase.
In the compositions disclosed herein, ionic crosslinking is present within the alkenyl arene block domains within the modifier present in the polyester blend composition. The function of the ionic crosslinking within the modifier phase is not entirely understood as it pertains to the properties of the blend composition.
The neutralization effect herein is to be distinguished from ionic crosslinking as is disclosed in Rees, U.S. Pat. No. 3,264,272; Saito et al., U.S. Pat. No. 4,429,076; and Gergen et al., U.S. Pat. No. 4,578,429. Rees and Gergen et al. ('429) utilize ionic crosslinking solely to improve the properties of the pure hydrocarbon polymer as opposed to improving the properties of polyester blend compositions.
Rees is limited to ionic crosslinking in homopolymer systems in which the carboxyl groups are distributed throughout the homopolymer molecule. As such, Rees does not deal with copolymers and resulting alkenyl arene domain formation. On the otherhand, though Gergen et al. ('429) addresses block copolymers, the carboxyl groups are distributed throughout the elastomeric diene block rather than the alkenyl arene blocks.
Saito et al. utilize ionic crosslinking to improve the properties of modified block copolymer which are to be blended with a thermoplastic polymer having a polar group thereby improving the impact resistance and hardness of the blend. In Saito et al., the block copolymer is modified by grafting maleic anhydride onto the conjugated diene portion thereof.